The dopaminergic agonist, bromocriptine, has been studied in 50 patients with idiopathic parkinsonism. Controlled observations indicate bromocriptine to be an active therapeutic agent, somewhat superior to others currently available. It is particularly helpful in patients experiencing certain adverse reactions to levodopa (L-dopa), such as the "on-off phenomena". Bromocriptine appears to be well tolerated, with maximum benefit achieved when combined with low doses of L-dopa. Administration of bromocriptine altered the metabolites of dopamine and serotonin in the cerebrospinal fluid (CSF). Concentrations of homovanillic acid were reduced, implying diminished presynpatic dopaminergic activity, while levels of 5-hydroxyindoleacetic acid were increased, suggesting blockade of postsynaptic serotonergic receptors. Attempts to potentiate the action of bromocriptine by inhibiting phosphodiesterase with caffeine proved unsuccessful. Bromocriptine was also administered in a controlled study to 6 patients with Huntington's chorea, following a recent report of therapeutic benefit from dopaminergic activation in this disease. However, rather than providing improvement bromocriptine was found to induce exacerbation. This finding supports the hypothesis that choreatic movements correlate with increase activity of dopaminergic systems.